1. Field of the Invention
The present invention relates generally to pre-impregnated composite material (prepreg) that is used in making high performance composite parts that are especially well-suited for use as aerospace components. More particularly, the present invention is directed to novolac-based epoxy resins that are toughened with thermoplastic materials and used as the resin matrix in such prepreg.
2. Description of Related Art
Composite materials are typically composed of a resin matrix and reinforcing fibers as the two primary constituents. Composite materials are often required to perform in demanding environments, such as in the field of aerospace where the physical limits and characteristics of the composite part is of critical importance.
Pre-impregnated composite material (prepreg) is used widely in the manufacture of composite parts. Prepreg is a combination that typically includes uncured resin and fiber reinforcement, which is in a form that is ready for molding and curing into the final composite part. By pre-impregnating the fiber reinforcement with resin, the manufacturer can carefully control the amount and location of resin that is impregnated into the fiber network and ensure that the resin is distributed in the network as desired. It is well known that the relative amount of fibers and resin in a composite part and the distribution of resin within the fiber network affect the structural properties of the part.
Prepreg is a preferred material for use in manufacturing load-bearing or primary structural parts and particularly aerospace primary structural parts, such as wings, fuselages, bulkheads and control surfaces. It is important that these parts have sufficient strength, damage tolerance and other requirements that are routinely established for such parts.
The fiber reinforcements that are commonly used in aerospace prepreg are multidirectional woven fabrics or unidirectional tape that contains fibers extending parallel to each other. The fibers are typically in the form of a bundle of numerous individual fibers or filaments that is referred to as a “tow”. The fibers or tows can also be chopped and randomly oriented in the resin to form a non-woven mat. These various fiber reinforcement configurations are combined with a carefully controlled amount of uncured resin. The resulting prepreg is typically placed between protective layers and rolled up for storage or transport to the manufacturing facility. Combinations of carbon fibers and an epoxy resin matrix have become a popular combination for aerospace prepreg.
Prepreg may also be in the form of short segments of chopped unidirectional tape that are randomly oriented to form a non-woven mat of chopped unidirectional tape. This type of prepreg is referred to as a “quasi-isotropic chopped” prepreg. Quasi-isotropic chopped prepreg is similar to the more traditional non-woven fiber mat prepreg, except that short lengths of chopped unidirectional tape (chips) are randomly oriented in the mat rather than chopped fibers. This material is commonly used as a sheet molding compound to form parts and molds for use in making parts.
The compression strength and tensile strength of a cured composite material is largely dictated by the individual properties of the reinforcing fiber and matrix resin, as well as the interaction between these two components. In addition, the fiber to resin volume ratio is an important factor. The compression and tensile strengths of a composite part are typically measured at room temperature under dry conditions. However, the compression strength and tensile strength are also routinely measured at elevated temperatures (e.g. 180° F.) under wet conditions. Many parts exhibit a significant drop in compression strength and/or tensile strength under such hot and wet conditions.
In many aerospace applications, it is desirable that the composite part exhibit high compression and/or tensile strength under both room temperature/dry conditions and hot/wet conditions. However, attempts to keep compression and tensile strengths constant under hotter/wetter conditions often results in negative effects on other desirable properties, such as damage tolerance and interlaminar fracture toughness.
Selecting higher modulus resins can be an effective way to increase the compression strength of a composite. However, this can result in a tendency to reduce damage tolerance, which is typically measured by a decrease in compressive properties, such as compression after impact (CAI) strength. Accordingly, it is can be difficult to achieve a simultaneous increase in both the compression and/or tensile strengths without deleteriously affecting the damage tolerance.
Multiple layers of prepreg are commonly used to form composite parts that have a laminated structure. Delamination of such composite parts is an important failure mode. Delamination occurs when two layers debond from each other. Important design limiting factors include both the energy needed to initiate a delamination and the energy needed to propagate it. The initiation and growth of a delamination is often determined by examining Mode I and Mode II fracture toughness. Fracture toughness is usually measured using composite materials that have a unidirectional fiber orientation. The interlaminar fracture toughness of a composite material is quantified using the G1c (Double Cantilever Beam) and G2c (End Notch Flex) tests. In Mode I, the pre-cracked laminate failure is governed by peel forces and in Mode II the crack is propagated by shear forces.
One approach to increasing interlaminar fracture toughness for parts made from carbon fiber/epoxy resin prepreg has been to introduce thermoplastic sheets as interleaves between layers of prepreg. However, this approach tends to yield stiff, tack-free materials that are difficult to use. Another approach has been to add thermoplastic particles to the epoxy resin so that a resin interlayer containing the thermoplastic particles is formed between the fiber layers of the final part. Polyamides have been used as such thermoplastic particles. It also has been known to include a thermoplastic toughening agent in the epoxy resin. The toughening agent, such as polyether sulfone (PES) or polyetherimide (PEI), is dissolved in the epoxy resin before it is applied to the carbon fibers. Thermoplastic toughened epoxy resins, which include a combination of both thermoplastic toughening particles and a thermoplastic toughening agent, have been used in combination with carbon fiber to make aerospace prepreg.
The epoxy resin matrix may include one or more types of epoxy resin. It is known that various combinations of different types of epoxy resins may result in a wide variation in the properties of the final composite part. The curing agent used to cure the epoxy resin matrix can also substantially affect the properties of the final composite part. When formulating an epoxy resin for use as the resin matrix in aerospace prepreg, it is difficult to predict if a new combination of epoxy resin types and curatives will provide the desired combination of properties required for aerospace parts. This is especially the case when a thermoplastic toughening agent and thermoplastic particles form part of the epoxy resin formulation. Accordingly, there is a great deal of testing involved when one attempts to formulate new thermoplastic toughened epoxy resins in order to determine if the resin is suitable for use as resin matrix in aerospace prepreg.
Although existing aerospace prepregs are well suited for their intended use in providing composite parts that are strong and damage tolerant, there still is a continuing need to provide aerospace prepreg that may be used to make composite parts that exhibit desirable combinations of compression/tensile strength, damage tolerance and high interlaminar fracture toughness.